This invention relates to glass compositions and articles made therefrom which are essentially transparent to visible light and are capable of being ion exchange strengthened at an exceptionally high rate of speed. Moreover, the novel glasses of the present invention are rendered suitable for large scale production and subsequent use in automotive, aircraft, architectural, tableware and other applications by virtue of their low melting temperatures and high degree of resistance to chemical attack.
Ion exchange strengthening (or "chemical tempering") of glass involves an exchange of ions near the surface of the glass article with ions from an external source, typically a molten inorganic salt bath, the object being the generation of a zone near the surface of the glass which is in a state of compression relative to the interior portions of the glass. There are two types of ion exchange strengthening which differ substantially in theory and operation. The first type of ion exchange treatment is carried out above the strain point of the glass and has as its object the alteration of the glass composition at the surface so as to lower the thermal coefficient of expansion in the surface layer. As the glass is cooled, a compressive stress develops at the surface due to the expansion differential. This approach was taught by Hood and Stookey in U.S. Pat. No. 2,779,136. The second type of ion exchange strengthening is characterized by treatment below the strain point of the glass, wherein surface compression is generated by substituting large ions from an external source (e.g., a molten salt bath) for smaller ions in the glass. Typically, the substitution is of sodium or potassium for lithium in the glass, or of potassium for sodium in the glass. The below-the-strain-point technique was first taught by Weber in U.S. Pat. No. 3,218,220.
Of the two types of ion exchange strengthening, the second (below the strain point) type is preferred for large-scale commercial use. This is because maintaining the glass below its strain temperature avoids causing undesirable distortion defects in the glass. Furthermore, since it is costly to include lithium in a glass as a batch ingredient, and because greater strengthening can generally be achieved, it is desirable that sodium, rather than lithium, be the ion in the glass which is replaced. In that case, the larger ion which enters the glass is most advantageously potassium. Hence, this invention is directed specifically to the improvement of ion exchange strengthening processes which involve replacing sodium with potassium below the strain point of the glass.
Conventional soda-lime-silica flat glass compositions can be strengthened by ion exchange, but the great length of time required to produce a significant compression layer depth is incompatible with many high volume commercial operations. For this reason, special glass compositions have been developed which have greatly enhanced ion exchange properties, chief among which are the Al.sub.2 O.sub.3 and/or ZrO.sub.2 containing glasses disclosed by Mochel in U.S. Pat. Nos. 3,485,702; 3,752,729; and 3,790,430. Variations of these alumina or zirconia containing glasses may be seen in many U.S. patents including the following (some of which contain lithium):
U.s. pat. No. 3,357,876 -- Rinehart PA1 U.s. pat. No. 3,433,611 -- Saunders et al. PA1 U.s. pat. No. 3,481,726 -- Fischer et al. PA1 U.s. pat. No. 3,485,647 -- Harrington PA1 U.s. pat. No. 3,498,773 -- Grubb et al. PA1 U.s. pat. No. 3,772,135 -- Hara et al. PA1 U.s. pat. No. 3,778,335 -- Boyd PA1 U.s. pat. No. 3,844,754 -- Grubb et al.
While these specially adapted ion exchange glass compositions of the prior art greatly reduce the amount of time required for ion exchange treatment compared to conventional soda-lime-silica glass, the commercial use of lithium-free ion exchange glasses remains limited to low volume specialty items because treatment times are still impractically long for many applications. Moreover, many of the prior art compositions have melting temperatures considerably higher than soda-lime-silica glass and thus are not readily adapted for use in existing melting and forming facilities. Thus, it would be highly desirable to have glass compositions available which could be more rapidly strengthened by exchange treatment with potassium and at the same time be possessed of melting temperatures more in line with ordinary soda-lime-silica glass. Other factors such as transparency, chemical durability, and the cost of raw materials also must be taken into consideration.
Substantial progress toward the above-noted goals was attained by the glass compositions disclosed in parent application Serial No. 605,108, the disclosure of which is hereby incorporated by reference. The present invention represents an even greater improvement over those glasses, particularly in regard to the speed with which a deep compression layer can be created in the glasses.